1. Field of the Invention
This invention relates to ink compositions useful for ink jet printing. Preferably, these compositions, when printed, promote improved waterfastness.
2. Discussion of the Related Art
Ink jet printing is a non-impact process wherein droplets of ink are produced and deposited on a substrate such as paper, transparent film, or textile material in response to an electronic signal. Typical ink jet printing systems are continuous stream or drop-on-demand type.
In continuous ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the breakup point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field. The field adjusts the trajectory of each droplet to direct it to a gutter for recirculation or to a specific location on a recording medium to create images. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium by pressure generated in accordance with digital data signals.
There are three basic types of drop-on-demand ink jet systems. One type has an ink filled channel with a nozzle on one end and a piezoelectric transducer which produces pressure pulses.
A second type of drop-on-demand system is known as a thermal ink jet or bubble jet printer. The major components are an ink-filled channel with a nozzle at one end and a heat generating resistor. A pressure difference between the inside and outside of the channel is generated by heating the resistor, causing the ink in the immediate vicinity to vaporize, creating a bubble.
The third type of drop-on-demand ink jet system is known as an acoustic ink printer. This printer utilizes a piezoelectric transducer to form acoustic energy which exerts pressure on the liquid in the channel, resulting in the ejection of small droplets of ink onto an imaging substrate.
Conventional inks for ink jet printers generally comprise a dye which is soluble in a vehicle of water or a mixture of water and a water-soluble or water-miscible organic solvent. However, dyes have several disadvantages when used in ink jet inks. Dyes, being water-soluble or soluble in a water and organic mixture, may redissolve and run when exposed to moisture or water, i.e., they have poor waterfastness. Dyes may also be sensitive to changes in color or pH. Dye images may further smear and rub off on contact with felt pen markers or upon being rubbed with a finger. Dyes also exhibit poor light stability when exposed to visible or ultraviolet light, for example, sun or fluorescent light. Inks comprising soluble dyes may also experience clogging of the jetting channels due to solvent evaporation, thereby resulting in changes in the dye's solubility, dye crystallization, and the concentration of impurities. Dye-based ink may also exhibit poor thermal and chemical stability which can result in poor print quality. The dye may also bleed or diffuse into pores of the paper, thus causing poor print quality and low optical density of the image. Because of these problems, specialty paper is often necessary for ink jet inks containing a dye. Similarly, the addition of polyethyleneimine ("PEI") to certain water-soluble dyes in inkjet inks is known to improve the waterfastness of the dyes on the receiving paper. Furthermore, undesirable heavy metals or other additives may be necessary in dye-based inks to obtain an infrared or readable image. For a general discussion on the properties, preparation, and uses of aqueous inks, see The Printing Manual, 5th Ed., Leach et al., Eds. (Chapman & Hall, 1993), incorporated in its entirety by reference.
While these dye-based inks containing PEI constitute an improvement, they still have disadvantages. The dyes of the inks still tend to exhibit several undesirable properties, as discussed above. In addition, the desired waterfastness is not always achieved.
Although not as common, pigments have also been used as colorants in inkjet inks, either as alternatives for, or in combination with, dyes. Pigments tend to offer improved properties to dyes in areas such as lightfastness, image density, thermal stability, pH insensitivity, oxidative and chemical stability, non-bleeding, compatibility with other colorants, and compatibility with both coated/treated and plain papers. Pigments used in ink jet inks include carbon black, titanium dioxide white, cobalt blue (CoO-Al.sub.2 O.sub.3), phthalocyanine blue, phthalocyanine green, and chrome yellow (PbCrO.sub.4).
Regardless of their desirable properties, pigments have not replaced dyes as the prevalent colorant in ink jet inks because of the difficulty often associated with obtaining an adequate dispersion of pigment in an ink. Pigment particles such as carbon black generally exist in a clumped or agglomerated state. To prepare ink jet inks, however, the carbon black should be dispersed and then stabilized in that dispersed form. Such dispersion and stabilization are difficult to attain. Unstable or inadequate dispersion may result in scaling or clogging of the opening, thereby interrupting nozzle flow. As such, the extent of dispersion directly affects ink jet printing characteristics such as ejectability, print quality, optical density, and the like.
Thus, there remains a need for improved ink compositions possessing the required stability, especially for use in ink jet printers, which overcome the problems typically associated with current pigment and dye-based systems. In addition, a need remains for ink composition generating print images having improved waterfastness.